Advancements in the design of Organic Light Emitting Diode (OLEDs) displays, such as Active Matrix OLED (AMOLED) displays, have resulted in an increase in the variety of applications that incorporate such display technology. Unlike many other types of displays, such as conventional backlit LCD designs, AMOLED devices include light emitters in each individual pixel and require no backlight. These individual pixels emit light with intensity according to a value programed into that pixel, which causes a proportional electrical current to be supplied to the in-pixel OLED device. This OLED current (IOLED) is controlled by circuits associated with each pixel, which may include one or more thin film transistors. (TFT). In other types of displays that use a backlight to create the light that is emitted by the display, the display brightness is able to be easily adjusted by simply changing the intensity of light emitted by the backlight of the display. In contrast to adjusting one brightness value that controls the backlight intensity for the entire display, adjustment of brightness in an AMOLED display is accomplished by modifying the intensity of light emitted by each OLED element in the display.
Controlling display brightness is often used to control power consumption, whereby the brightness of light emitted by the display is varied in response to ambient light brightness and also in response to the content that is being displayed. In displays with a common backlight, such as conventional Liquid Crystal Displays (LCDs), algorithms such as content aware brightness/backlight control (CABC) reduce power consumption by determining limits on pixel brightness based upon an analysis of the data defining all of the pixels of the displayed image. In general, displays with content aware brightness control (CABC) are controlled by pulse with modulation (PWM) of the backlight based upon an analysis of the backlight brightness required by the image being presented on the display.
The brightness of an entire AMOLED display is able to be controlled globally by controlling the time that each pixel emits light, which is referred to as “emission time,” or by controlling, e.g., limiting, the electrical current delivered to the pixel OLED element during the emission time. Limiting emission time is able to reduce pixel brightness by shortening the duration by which all elements of the OLED display are in a light emission phase. In one example, a switch is placed between the drive transistor of the pixel and the OLED element of the pixel opens after the display has been configured to have each element emit light at its programmed intensity. That is to say, the switch, which is able to be implemented as a Thin Film Transistor (TFT), is pulsed and the OLED element will only emit light when the switch is closed.
Lowering the brightness of all pixels of an AMOLED display is able to be performed by dynamically changing the voltages of the DC power or bias lines supplying all OLED pixel elements. In one example, the two polarities of direct current (DC) power lines supplying power to the pixels of a display are indicated as EL_VDD and EL_VSS. Lowering the voltage between EL_VDD and EL_VSS causes a reduction in the voltage across the OLED pixel and thereby reduces the electrical current passing through the pixel and thereby lowers the brightness of the entire display.
In order to achieve desired aesthetics when performing the above described brightness control mechanisms, an analysis of the relationships of the intensity of each pixel in an image to be displayed is performed in order to determine an effective amount of overall display brightness reduction given the image data to be displayed. In general, specialized circuitry or other image processing resources are used to perform this image frame data analysis. Such additional processing adds complexity to the associated display driver or controller circuitry of a display.
Therefore, the operation of circuits used to reduce display brightness in active matrix displays with emissive pixel elements increases the cost and complexity of such circuits, thereby limiting the inclusion of energy conserving brightness control circuits in such displays.